In the United States most automotive vehicle bodies are manually welded on continuous flow carrousel gate lines. Different loosely assembled bodies may be prescheduled to move through the line on pallets or trucks with matching side locating "framing" gates, manual welding being employed as required for whatever body is moving on the line.
Some body assembly plants employ framing bucks which hold all main components--underbody, sides and top--in proper relation in a single special fixture, with manual welding likewise employed to accommodate different welding requirements for any particular body. In some cases automatic multiple spot welders are mounted on a framing head with pivoting pillars; however, in such cases the weld locations are fixed for each particular body with no provision for programming welding heads for different bodies.
In one case for a particular car model having three body styles, programmable robots were provided for respot or finish welding at successive fixed stations. Provision was made for initial framing of the main components in a first carrousel section where trucks carrying the underbody move continuously along with matching framing side gates for the individual body styles, the welding being performed partially by hand and partially by automatic welding heads limited to common weld spots for all three body styles with the continuous flow trucks returning to starting position at the end of each weld cycle. Hand welding was employed for any spots which differed for the various body styles. The robot welding programmed for automatic change for the different body styles all takes place in the second section after the body geometry is fixed in the first continuous flow carrousel framing station. Following the intermittent flow through the second robot section provision was made for the trucks to resume continuous flow through a third section where finish respot welding was manually performed.
In the case of automatic lines the transfer system is always a rigid system, which requires fixed steps from each welding station to the next one and a necessary route of the parts with a preset and constant speed of the transfer.
The disadvantage of this known system consists specifically in its rigidity, which makes extremely difficult and expensive the possible introduction or elimination of supplementary or intermediate operations and makes it necessary to rebuild all tooling for the production of a new model of car which must go through the same line.
With regard to major sheet metal vehicle body sub-assemblies, such as side apertures or doors, it is conventional practice to manually assemble component parts in a single purpose multi-spot press having no programmable robot features. The parts are accurately located by single purpose fixtures which typically provide welding of up to thirty spots per station. With such approach it is, of course, necessary to provide a complete multi-spot press with all necessary welding heads for each different multi-part sub-assembly with resultant high equipment costs for model changeover as well as production time delays in the order of four to six weeks.